Many biochars have a complex carbon lattice structure with aromatic and aliphatic domains, acidic and basic groups, vacancies, metallic and non-metallic elements, and free radicals. Biochars also have separate mineral oxide, silicate and salt phases, and small and large organic molecules. In the rhizosphere, such constituents can be involved in chemical and biological processes along a soil–microbe–plant continuum, including nutrient cycling, metal chelation and stabilization, redox reactions, and free radical scavenging. It is hypothesized that the greater the amount of these nanoparticles and dissolved components, the greater will be plant and microbial responses. We provide suggestions for developing low-dose, high-efficiency biochar–nanoparticle composites, as well as initial field trial results and detailed characterization of such a biochar–fertilizer composite, to highlight the potential of such biochars.